Skitch

This is one I have talked about before, but am finding more and more usage for.  I can take a photo of a tool that a visually impaired kiddo uses and access it through the Skitch app.  Once the photo is there, you can draw on the photo, type on it, label parts of it, etc.  I have used it with maps too.  You can make notes on a map, draw a route, etc.  I can imagine it would be helpful in teaching a variety of concepts to non-visually impaired students.  It's free!